1. Field of the Invention
This invention relates to an improved vertical shaft type furnace construction, and burner design for use therein, which is particularly useful for continuously melting copper pieces such as cathodes.
2. Prior Art
Vertical gas-fired shaft type furnaces for melting metal, such as copper, are well known in the art. Examples of such furnaces are seen in: U.S. Pat. Nos. 3,199,977; 3,701,517; 3,715,203; 3,788,623; and in the prior art patents cited in each of them.
Generally these furnaces have a substantially cylindrical shape and are elongated in a vertical direction. The metal to be melted, such as copper cathode pieces having a low oxide content, is charged into the furnace from an elevated position. The metal drops toward the bottom of the furnace, where a plurality of burners inject hot gases into the melting chamber to cause the metal to melt. The molten metal is drained from the furnace by a suitable outlet near the bottom in order to continuously supply the molten metal to a holding furnace or to a casting operation.
The burners are usually arranged in one or more rows surrounding the lower portion of the furnace, in order to define a melting chamber, and are directly affixed into the furnace walls. Each of a plurality of burners, all fed fuel from one common source, injects a fuel and air mixture into a melting chamber causing a highly turbulent flame to impinge on that metal directly adjacent each burner. Refractory tunnel type burners are known in the art as means for supplying a high temperature blast to a furnace. Typically, the throat mix type of burner is used in the prior art furnaces since they do not experience some of the problems common to a premix type burner such as backfires in the supply manifolds or flameouts, that is, isolation of the flame from the combustion ports. However, the throat mix burners of the prior art have disadvantages also. Throat mix burners must have a very turbulent high velocity flame to ensure adequate mixing of the fuel and air in the short space allotted within the burner. This results in a high operating noise level and very severe service conditions which deteriorate the furnace and burner refractories. When the deterioration reaches a certain state the operating efficiency of the burner and furnace is so adversely affected that reconditioning is required. Specifically, the deterioration has resulted from spalling, slagging, abrasion, or some combination of these. Spalling may be defined as the physical break-down or deformation or crushing of the refractory attributed to thermal or mechanical or structural causes. Slagging is the destructive action that occurs in the refractory due to chemical reactions occuring at the elevated temperatures involved. Abrasion is considered to be the deterioration of the refractory surfaces by the scouring action of solids moving in contact therewith. The solids may be carried by or formed in the combustion gases.
It is generally considered that in the most efficient types of refractory tunnel burners the refractory has good insulting properties, high heat resistance, and a rough interior surface texture. After the burner is lighted the refractory is heated and thereafter serves to maintain ignition. The roughness of the refractory surface causes the gases flowing adjacent thereto to be slightly turbulent and therefore exert a catalytic effect upon and consequently accelerate the combustion process. However, refractories which have good insulating properties and a rough surface also tend to have less resistance to the abrasive effects of the high velocity combustion gases and therefore experience much faster wear than a more dense, smooth refractory, such as silicon carbide. Another disadvantage of prior art burner arrangements is that when the combustion products are not properly mixed within the burner and before entering the furnace they have an uneven, unpredictable effect on the melting process, especially when operated over a varying range of melting rates which is necessary when supplying molten copper to a variable rate continuous casting system.
In summary, the main problem heretofore encountered with the prior art vertical furnace and burner combinations is that it is sometimes metallurgically unsuccessful when adapted to melt copper cathodes and is used to supply molten copper to a continuous casting and rolling process which is intending to produce electrical conductor grade copper bars. Part of the problem is that the molten copper becomes contaminated with unacceptable amounts of impurities. For example, oxygen and sulphur which are easily introduced into the molten metal from the combustion process, have a detrimental effect on the subsequent rolling of the cast copper into bars. Also, slags and metallic contaminants can be introduced into the melt which thereafter have a detrimental effect on the quality or conductivity of the final product. Thus, although vertical furnaces and various types of burners are well known in the art, significant needed improvements therein have been made by the present invention.